WASHINGTON (Reuters) - A strange-looking dinosaur with rows of tiny teeth crammed into the very front of its jaws and fragile air-filled bones may have been the "cow of the Mesozoic," and far more common than better-known dinosaurs, scientists said on Thursday.

Its shovel-shaped jaws and tightly packed teeth -- up to 10 rows of teeth -- allowed Nigersaurus taqueti to vacuum through ferns and other ground cover, a team led by Paul Sereno at the University of Chicago reported.

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The researchers used a combination of computer modeling, X-rays and good old-fashioned digging to build a model of an elephant-sized animal that lived 110 million years ago in what is now the Sahara desert in Niger.

And they argue that their work calls into question some of the standard models of other related dinosaurs such as Diplodocus that show the creatures standing and browsing in the treetops.

Such creatures, known as diplodocids, more likely grazed, Sereno writes in his report, published in the Public Library of Science journal PLoS ONE at http://www.plosone.org/doi/pone.0001230.

Enough bone was found to show the orientation of the semicircular canals -- the organ of equilibrium. What they found astonished the researchers.

"In everyday life this animal had its nose pointed towards the ground," Sereno told a news conference at the National Geographic Society, which helped pay for his 10-year study of the 30-foot-long (10 meter) Nigersaurus.

What this says, argues Sereno, is that experts have long mistakenly believed that many herbivore dinosaurs behaved like living long-necked animals such as giraffes.

"I think we were blindsided and missed the cows of the Mesozoic," Sereno said.

FERN MOWER

"We know from its brain that its head was down and we know from the neck that it operated like a boom," he added. "This was some kind of a fern mower the likes of which we had never seen."

Fossilized pollen and other evidence show Africa was covered with ferns and other lush, low-growing plants. "Would there be some parallel of a cow in the Mesozoic or would it all go uneaten?" he asked.

"We think the neck was simply for reach. It could simply sit there and mow a large area."

The teeth were packed so tightly into rows that the top and bottom jaws acted like two single blades of a pair of shears.

The researchers sawed a few teeth open. "You can actually see the daily increments of dentin," Sereno said. Nigersaurus was losing its teeth every 30 to 35 days, faster than any dinosaur known.

But another tooth was always waiting behind the lost tooth.

Sereno believes Nigersaurus and similar grazers were once the most common dinosaurs, but their fossils have been too fragile to have survived to be discovered.

How does he know? Teeth. Similar teeth have been found in Europe, South America and in several parts of Africa.

The vertebrae are hollow and the skull is so delicate that only bits and pieces remain from most of the animals. "It doesn't take much to weather these away," the University of Chicago's Bob Masek, who prepared the bones, said in an interview.

"The skull is made light so the neck doesn't have to support a lot of weight," Masek said.